Among the discharge lamps, high intensity discharge lamps (HID) such as metal halide valves, high pressure sodium valves and mercury valves have advantages such as large luminous flux, high lamp efficiency and long life. Thus, they have been used as lights in indoor or outdoor facilities, warehouses or factories, or as street lights. Recently in particular, they have been used as headlights of vehicles such as automobiles. To turn on such a discharge lamp, it is necessary to apply a prescribed voltage to the valve at the start, and to superimpose a high voltage start pulse thereon. Thus, the ballast apparatus includes a stabilized DC power supply circuit (DC-DC converter or the like) for lighting the discharge lamp stably, an inverter circuit (AC power supply circuit) for converting the DC voltage to a rectangular wave AC, and anigniter (start circuit) for generating a high voltage starting pulse. In addition, to bring the DC power supply circuit and inverter circuit into operation, their power is supplied from the DC power supply such as a battery. In this case, the power supply must be controlled to prevent an overcurrent. As conventional discharge lamp ballast apparatuses that can prevent the overcurrent, the following examples are known.
A first conventional example relates to a technique of fail-safe operation in case of a ground fault of the electric wiring of a discharge lamp due to some failure. It has a discharge lamp current detecting resistor provided across a low voltage common terminal of an H bridge circuit constituting an inverter circuit and a ground (GND). If the ground fault occurs, the current detecting resistor detects it, and the switching transistors (MOS transistors) constituting the H bridge circuit are turned off in response to the detection, thereby preventing the overcurrent from the DC power supply (for example, see Relevant Reference 1).
A second conventional example relates to a technique for limiting the power supply current to prevent an excessive increase of the power supply current when the DC power supply voltage decreases. It includes a DC power supply circuit (DC-DC converter) using a transformer of the DC/DC converter which has a primary winding and secondary winding isolated from each other; a current detecting resistor provided on the primary winding side for detecting the input current (power supply current) to the DC power supply circuit; and a current limiting controller for limiting the input current to the DC power supply circuit in response to the detection signal, thereby preventing the overcurrent from the DC power supply (for example, see Relevant Reference 2).
Besides the foregoing examples, the following Relevant References 3-6 are enumerated as conventional techniques relating to the power supply in the discharge lamp ballast apparatuses.
Relevant Reference 1
Japanese patent application laid-open No. 2001-43989.
Relevant Reference 2
Japanese patent application laid-open No. 7-169585/1995.
Relevant Reference 3
Japanese patent application laid-open No. 6-188078/1994.
Relevant Reference 4
Japanese patent application laid-open No. 9-223590/1997.
Relevant Reference 5
Japanese patent application laid-open No. 2002-110384.
Relevant Reference 6
Japanese patent application laid-open No. 2003-323992.
The conventional discharge lamp ballast apparatuses are configured as described above. In particular, the first conventional example relates to the technique of preventing the overcurrent at the ground fault, which does not handle the factors other than the ground fault such as the voltage reduction of the DC power supply as in the second conventional example. The boosting transformer of the DC/DC converter used by the DC power supply circuit (DC-DC converter) of the first conventional example is a step-up autotransformer. The step-up autotransformer is preferable as the DC-DC converter because it has a smaller winding and a smaller core. Accordingly, if the first conventional example could include the current detecting resistor between the DC power supply and the ballast apparatus in the same manner as the second conventional example, the first conventional example would be able to achieve the effect of the second conventional example, which would be vastly preferable. However, when the DC/DC converter uses the step-up autotransformer as its transformer, the load current from the DC power supply flows not only through the DC power supply circuit, but also to the post-stage inverter circuit and the like, which differs from the second conventional example that uses the DC/DC converter transformer having the primary winding and secondary winding isolated from each other. In other words, the load current from the single DC power supply is split to a plurality of load circuits such as the DC power supply circuit and inverter circuit. Accordingly, even if the current detecting resistor is provided between the DC power supply and the ballast apparatus as in the second conventional example, the potential on the load side of the current detecting resistor changes with the current. Because of the changes, the voltage across the resistor for detecting the output current flowing through the inverter circuit varies, and this presents a problem in that the power supply current cannot be detected accurately.
The present invention is implemented to solve the foregoing problem. Therefore it is an object of the present invention to provide a discharge lamp ballast apparatus capable of detecting the power supply current fed from the single DC power supply accurately, and capable of controlling the current fed from the DC power supply in response to the detection result in the discharge lamp ballast apparatus with a configuration in which the load current from the single DC power supply is split to a plurality of load circuits such as the DC power supply circuit and inverter circuit as in the case where the step-up autotransformer is used as the transformer constituting the DC power supply circuit (DC-DC converter).